Toxoplasma gondii is a wide-spread opportunistic pathogen causing severe encephalitis in AIDS patients. T. gondii is also an experimental model system for the study of related apicomplexan parasites important in AIDS including Cryptosporidium, Cyclospora, Isospora, and Plasmodium the causative agent of malaria. Replication of apicomplexan parasites takes place only inside the cells of its host, within a specialized compartment formed during invasion - the parasitophorous vacuole. Protein secretion is tightly associated with host-cell invasion and establishment/maintenance of the parasitophorous vacuole. As invasion is essential to parasite replication these steps might also pesent potential targets for anti-parasite drug or vaccine development. This proposal aims to study the role of protein secretion and targeting in host cell invasion and vacuole maturation. The secretory apparatus in T gondii is highly diversified, with secretion occurring from three sets of organelles: micronemes, rhoptries, and dense granules. We will characterize protein trafficking to and from these organelles using in vivo reporter genes. Sequence mapping using deletion analysis and more subtle point-mutations will permit us to establish the molecular signals involved in this sorting process. Conditional mutants will be isolated in a phenotypic screen enriching for the accumulation of a secretory reporter. Isolated mutant parasites will be used to establish if secretion is essential or coincidental with the invasion event. Mutant analysis will also provide critical information towards the function of the individual secretory organelles involved.